Gold investors returning from their summer holidays will be eager to see whether the precious metal can sustain its record-breaking rally, or if it will succumb to the curse of September.

Bullion has dropped every September since 2017. Over that period, the average decline has been 3.2% in September – easily the worst month of the year, and far below the monthly average gain of 1%.

It’s a phenomenon that’s perplexed economists who believe markets should behave more efficiently, and it isn’t limited to gold: September is also commonly the worst month for US stocks, with average declines of more than 1.5% in the S&P 500 over the past decade.

The dynamic is far from reliable — gold has actually risen in September over a three-decade horizon — but one explanation for the recent weakness is that traders are buying bullion to take a defensive position over the increasingly turbulent summer months, before selling on their return to the office in September.

“Before you go on vacation and get away from your screens, you want to hedge the risk that you have in the market, and one way you can do that is to buy gold,” said Boris Mikanikrezai, an analyst at FastMarkets.

Academics have shown that some investors do “switch off” over the summer, and adding safe-haven bullion to the portfolio might offer peace of mind during a period that’s traditionally more volatile. Throughout history, conflicts and market meltdowns have broken out frequently over the summer, and volatility can be exacerbated when trading desks are understaffed and senior executives are away.

The flip side is that when September arrives, there’s an inbuilt headwind for gold. September is also traditionally the dollar’s strongest month, which means traders using other currencies can buy less gold with their money.

The precious metal has rallied 22% this year, including 8% since July. It has been supported by robust purchases by central banks, increased haven demand amid geopolitical tensions, and healthy buying of physical bars in the over-the-counter market.

Gold’s gains have also been driven by expectations the Federal Reserve will start to ease monetary policy next month. Fed Chair Jerome Powell said last week that the “time has come” to lower interest rates, but the speed and magnitude of cuts may be key to determining whether bullion maintains its momentum.

Whether these tailwinds are enough to break the September curse is another question.

“Seasonality points to a potentially challenging month ahead,” said Ole Hansen, head of commodities strategy at Saxo Bank.